Heat exchangers for two fluids are known whch comprise double-wall tubes, the inner wall having a smaller diameter being adapted to come into contact with one of the fluids by its inner surfce and the outer wall having a larger diameter being adapted to come into contact with the other fluid by its outer surface.
Although the two walls are mounted one inside the other with a very small mechanical clearance, a gas can nonetheless travel in most cases between these two walls so that, in the event of a perforation of one of the two walls, the fluid in contact with the perforated wall may spread and travel in the space between the two walls. It is thus possible to detect possible leakages by the perforation of one or the other of the walls by connecting the space of very small size between the walls to a detecting chamber.
The double wall therefore improves the safety of the exchanger since a double barrier is interposed between the two fluids and a leakage through one of the walls may be detected before the integrity of the other is affected.
Such double-wall tubes are employed, for example, in the case of steam generators of fast neutron nuclear reactors, the exchange fluids of which are liquid sodium and water which is vaporized by means of the heat conveyed by the liquid sodium. The liquid sodium at high temperature (500.degree. C. or more)comes into contact with the outer surface of the outer wall of the tube. The water and the steam under very high pressure and at a temperature in the neighborhood of that of the liquid sodium flow inside the tube in contact with the inner surface of the inner wall.
In this application to steam generators of fast neutron reactors, the fact use of double-wall tubes increases the cost of construction of the apparatus, but safety is increased in considerable proportions.
The use of steam generators having double-wall tubes even permits the elimination of the secondary circuit of nuclear reactors to be envisaged, since the heating of the water and its conversion into steam may be achieved directly by the heat conveyed by the primary sodium. Indeed, the probability of the occurrence of a leakage becomes very small and the detection of a possible leakage is achieved in a reliable and very rapid manner in a steam generator having double-wall tubes. The probability of a leakage occuring and developing and resulting in a notable mixture of primary sodium and water or steam is therefore very small and practically nil.
The double-wall tubes for a heat exchanger may be made by different methods which impart thereto different thermal or mechanical characteristics.
A first method consists in passing the tubular walls one inside the other and then creating a tight fit therebetween by a mechanical pre-stressing, for example by a co-drawing thereof. In such tubes, the actual area of contact between the two walls represents only approximately 5% of the total area of the confronting surfaces of the two walls. The thermal transfer through the two walls is thus substantially reduced by the existence of the interface and is liable to decrease over time by a releasing of the pressure of contact. Further, there occur differential expansions of the walls, longitudinal sliding between the two walls and therefore wear of their interface. On the other hand,in the case of a perforation of one of the two walls, the fluid passing through the perforated or pierced wall and entering the interstice between the two walls can travel therein either throughout the length of the tube, or reach grooves machined in one of the contacting surfaces and thus reach the ends of the tube connected to a detection chamber. Indeed, although the interstice between the two tubes has a very small width, the fluid is stopped by no barrier since the actual surfaces of contact of the two walls have a small extent.
A second method consists in metallurgically interconnecting the confronting surfaces of the two walls, for example by brazing, the brazing metal covering the whole of the surfaces in contact of the two walls. In this way, there is a considerable improvememt in the thermal transfer, deterioration thereof over a period of time is avoided and longitudinal relative sliding of the walls of the tube when it expands is avoided. On the other hand, the detection of the leakages can theoretically only occur through grooves machined in one of the surfaces in contact of the two walls, and in the event of any leakage issuing from a perforation located between the grooves, it is highly probable that it will not reach them or reach them only after a very long period of time. Further, it is difficult to ensure that these grooves provide continuous passageways for the fluid after brazing.